The enzyme myeloperoxidase (MPO; donor: H2O2 oxidoreductase, EC1.11.7) is a well-established marker of myeloid differentiation. Most myeloid leukemias express MPO enzyme activity at the light microscopic level, whereas lymphoid leukemias characteristically lack such expression. However, the diagnostic significance of MPO RNA expression or of immunohistochemically detectable MPO protein expression in leukemic blasts is unclear. We studied the prevalence and diagnostic significance of MPO RNA and protein expression in 57 cases of MPO enzymenegative infant B-precursor acute lymphocytic leukemia (ALL), since the blast cells in this condition have been reported to show a high incidence of coexpression of myeloid-associated antigens. MPO expression was compared with other clinical and laboratory parameters. Of the cases examined, 56% showed detectable MPO expression at the RNA or protein level or both. Most positive cases showed MPO protein in many leukemic blasts, whereas a few cases showed substantial MPO protein expression in only a few blast cells. MPO expression showed no significant correlation with other markers of myeloid differentiation. Leukemic lymphoblasts in infant ALL frequently express MPO at the RNA or protein level; this expression does not imply an overall myeloid phenotype. The leukemic blasts in infant ALL may derive from an immature hematopoietic precursor cell not fully committed to lymphoid differentiation.